Joey Graceffa is seated onstage in front of a thousand shrieking teenagers. He's wearing an untucked white shirt, jeans and a bandanna around his neck. His eyes are a pellucid blue. He is 24, and in a few short years he has become one of the superstars of YouTube.

He's taking questions from Twitter. One girl wants to know, "How did you get such a nice butt?"

"Oh my God!" he squeals, looking at a picture on a phone. "It does look good!"

The crowd is desperate to know everything about him, to consume him. They're surging forward, hands in the air, emitting wild screams.

A boy, maybe 10 years old, steels himself and takes the microphone. "If you could be any animal," he says, "what kind of animal would you be?"

"A unicorn," Graceffa says, and his fans shriek their approval.

This is Vidcon. Some 20,000 teenagers have taken over the Anaheim Convention Center, playing arcade games and Quidditch, jumping into a pit of yellow plastic emoji balls and standing in long lines to meet their favorite YouTube celebrities.

"He's always happy," says Helen Keetley, 13, who came from Pennsylvania to get a picture with Graceffa. "When I'm upset, he makes me feel better."

She has come with a friend, Malia Nelan, and her friend's mom, who are from Virginia. This is actually the first time the two girls have met. They got to know each other on Twitter through their mutual love of Graceffa.

"He's responded to me 24 times," Keetley says.

"Five for me," Nelan says.

Backstage, Petar Mandich is staring into his phone. As Graceffa's manager, this is a standard pose for him. He's been in the background of so many Graceffa selfies, locked in communion with his iPhone, that someone made a meme out of it.

Joey Graceffa has had a big year. He surpassed 5 million subscribers, released his memoir and went on a book tour. He also came out as gay in a music video.

Photo by Gage Skidmore

Mandich has come to Vidcon prepared. In his backpack, he has a laptop, two Mophie phone chargers, four vanilla granola bars, two bottles of water, some aspirin and a pack of wipes.

"There's germs everywhere," he explains. "By day two it smells really bad. There's a lot of prepubescent people here who don't know they need deodorant. We were joking that Axe or Degree should be a sponsor. Just hand it out at the door."

Mandich, 27, also is carrying a purse, which belongs to Justine Ezarik, better known as iJustine, who is also his client. iJustine has been on YouTube since it started a decade ago. She has been with Mandich for five years — longer, she jokes, than most of her romantic relationships.

She's speaking on panels at Vidcon, interviewing people, giving interviews, signing her book and doing three social integrations for brand campaigns. It's a lot to keep up with, which is what Mandich is there for.

"He's my savior," she says. "He's my best friend."

Mandich's job, as he sees it, is to handle the business side of his clients' careers so they can focus on making content. Of course, anyone can upload a video on YouTube and build a mass audience without going through industry gatekeepers. But at the uppermost levels, where brand deals can run into five, six or even seven figures, having an entourage of agents, managers and publicists is essential to success.

It's an interesting moment to be in the YouTube business. YouTube debuted in 2005 with a democratic slogan, "Broadcast Yourself," but as the platform has matured, it has taken on the trappings of the traditional entertainment business. Within the community, there's a feeling that as their audiences grow up, YouTube stars are destined to take over Hollywood.

But that's not inevitable. If it happens, Mandich and people like him will have to make it happen.

The business of YouTube is complex, and the rules are still being written. Revenue can come from Google's AdSense, from endorsement deals with advertisers, from book or music sales, from merchandise or touring. Most professional YouTubers have some affiliation with a multichannel network, or MCN, which provides an ad sales force and some video production money in exchange for a slice of revenue.

MCNs also provide management to help shape a YouTuber's career. But as talent managers are quick to point out, that arrangement poses a conflict of interest. A manager who works for an MCN does not work for his client.

Some YouTubers have been burned by bad contracts, especially in the early days a few years ago.

"It's an industry that's dominated by children and inexperienced people," says Naomi Lennon, who runs a YouTube talent management company. "It hasn't been good for all the stars. There's a lot of exploitation. There's a lot of mismanagement of careers. There's a lot of false promises."

Mandich, Lennon and a few others are among a small cohort of independent managers who work exclusively for YouTube stars. Like managers in any other entertainment field, they take a commission based on a client's income. It's only in the last few years that revenues have grown enough to make managing YouTube stars a viable business.

"The cleanest way to do it ... is to represent them independently," says Sarah Weichel, another manager. "It's time for the industry to mature."

In part, a manager's job is to play matchmaker between the talent and the brands that want to advertise on YouTube. But the job is also to be a confidant and a life coach, helping clients shape their careers.

"It's a hard business," Anthony says. "It's talent. You gotta be there with them. They're needy. You gotta earn their trust."

Mandich, he says, "is the best at what he does in the business."

Mandich met iJustine while working at the Collective, a management company with clients in the music industry and on YouTube. Mandich, then 22, had studied music management at USC before joining the company. He and iJustine formed a close bond.

"We just hit it off," Mandich says. "We had a good understanding of her brand, and what she needs to build her business."

iJustine was already one of the biggest stars on YouTube. She had been posting videos for years, developing a persona as a tech-obsessed Apple fangirl. One of her early breakout hits was a video in which she pores over a 300-page iPhone bill. She was clever about it. She's playing a ditz, but she's in on the joke.

"She was incredibly marketable," says Richard Frias, who was her first manager. "And as opposed to traditional Hollywood talent, she was self-sustaining. She could create and market her own content."

At first Frias would struggle to put together endorsement deals for just a few thousand dollars. But by 2010, YouTube was becoming a bigger business. AT&T paid iJustine and a few other YouTube stars to make videos across the country to demonstrate the firm's nationwide network.

After a year, the Collective invested more heavily in YouTube, launching its own multichannel network. A trio of managers at the company, who did not want to be part of an MCN, left and founded an independent management company, Addition.

When Mandich left, he took iJustine with him.

While it might seem as if the world of YouTube entertainers would be as infinite as the Internet itself, it is, in fact, a very small community. The top YouTube stars appear in one another's videos — called a "collab" — which is a key way to boost viewership. Sometimes they live in the same house, or date each other, or get into feuds.

Thanks to his close relationship with iJustine, Mandich was able to sign other clients through word-of-mouth. They included Catrific, aka Cat Valdes, and Joey Graceffa.

In his memoir, In Real Life: My Journey to a Pixelated World, Graceffa writes about meeting iJustine at the first Vidcon in 2010: "She was still basically a celebrity to us." Graceffa and his friend waited in line to meet her and were thrilled when she recognized them from their videos.

Justine Ezarik, better known as iJustine, has been on YouTube since it started a decade ago and has been with Mandich for five years — longer, she jokes, than most of her romantic relationships.

Photo by Glenn Francis

"That's when everything started to really sink in," he writes. "We weren't there just as fans; we were an actual part of this fast-growing community."

Soon thereafter, Graceffa signed with Naomi Lennon.

"He is a star," she says. "He was doing music videos and parodies. He started doing them daily, and his channel exploded. He's a very good-looking guy and you're in a teen female audience."

Graceffa had big ambitions — he wanted to star in feature films and create his own TV series. Lennon helped sign him up at UTA, the agency that has moved most aggressively into YouTube world. She says it took six months to get the first meeting.

"I knew he was gonna be big way before they did," she says. "Now he's one of their biggest stars."

Ultimately, Graceffa and Lennon parted ways — she declines to go into specifics — and he found himself looking for a manager. He was introduced to Mandich.

"We had a big meeting about goals. I want to move into more scripted stuff and high-quality content," Graceffa says. "He's helped make that happen. He's giving me good advice. I just trust him so much."

Graceffa has had a big year. He surpassed 5 million subscribers, released his memoir and went on a book tour. He also came out as gay in a music video. The release of the video was timed to the launch of the book, and it was all carefully orchestrated with Mandich at UTA.

"He wanted to create some content all along to share his story and empower his audience and fans who might be struggling with identity and confusions," Mandich says. "It was very well received."

The music video recently surpassed 10 million views.

Graceffa had been under some pressure to come out, ever since YouTube star Connor Franta came out in December. Franta's book, A Work in Progress, was released in April by the same publishing imprint, Keywords Press, that released Graceffa's book. Keywords, a partnership between Simon & Schuster and UTA, also released I Hate Myselfie, a memoir by YouTuber Shane Dawson, who came out as bisexual in July.

Mandich says no matter what the haters say, Graceffa's message of self-acceptance resonated. At his book tour stops, teenage boys would often come out to him.

"People can assume what they want," Mandich says. "It's a very personal thing he did, and the way he did it was creative and artistic."

Graceffa's fans seem to approve.

"I was so proud of him," Helen Keetley says. Her friend Nelan agrees: "We were all so proud."

Backstage at Vidcon, a coordinator at the signing hall is going over some last-minute details with iJustine.

"So autographs but no selfies," the woman says.

"We can do selfies," iJustine says. "The selfie is the new autograph."

"OK, great," the woman says. "I'll just brief your people over there, because they're trained to jump in on anything."

iJustine is signing her book — I, Justine: An Analog Memoir, also from Keywords Press — for several hundred lucky fans who won the chance to meet her through Vidcon's signing lottery. She has been on book tour and has the routine nailed: hug, selfie, repeat.

In the next line over, Graceffa is signing his book and handing out posters. Some fans are in tears. Their hands instinctively go up to their faces, and someone has to tell them to put their hands down so they'll look good in their photo. Some are wearing homemade Graceffa-themed clothing. A few have elaborately scripted this moment, and their mothers are standing by to shoot video.

While this is going on, a woman from YouTube leads a parade of brand executives through the signing hall. One has a badge indicating she is from Lexus. The executives are bearing witness to the power of YouTube.

A marketing executive from Urban Outfitters is there with her daughter. The daughter knows who iJustine is, and gets a selfie. Mandich takes the woman's card — a potential prospect.

For Mandich, the key to a good brand deal is that it has to be "organic." The YouTube star has to have some authentic love for the brand, or the audience will smell a rat.

Petar Mandich and his colleagues are focused on what he calls "multi-threat talent" — vloggers who can do other things besides making videos.

PHOTO BY DANNY LIAO

A classic move in YouTube campaigns is for the brand to play the hero. The YouTube star wants to make an awesome video but doesn't have money for a high-quality production. So the brand pays for the video, the YouTube star gets to be in it, and the audience gets to watch it.

"It's win-win-win," Addition's Anthony says.

The fans are typically rooting for the star's career, and see a high-quality branded video as a great career move.

It also helps if a YouTube star is "brand-friendly." Both iJustine and Graceffa are pretty wholesome performers. They're upbeat. They don't swear. They don't do anything truly racy.

Mandich often is able to package Graceffa and iJustine together to launch a brand campaign. In 2014, both made videos for H&R Block. Graceffa's was a music video in which he demonstrated fun things you can buy with a tax refund. It racked up 50,000 likes and 500,000 views, and the comments were overwhelmingly enthusiastic.

Said one: "THAT WAS AMAZING IM FANGIRLING SO OMG I CANT BELIEVE YOU WROTE THAT OML DYING"

Another: "This should be on TV."

More recently, Graceffa and iJustine both wore red noses in a campaign for Walgreens.

"They wanted a huge social and video play," says Andi Poch, of Townsquare Media Group, which ran the campaign. "My first call is always to Petar. ... He tells me who would be right and wrong for something, and he'll tell me when and how to best use that talent."

Many YouTube stars are managed by their parents or a friend. So it's refreshing, Poch says, to work with someone who knows how the business works, can negotiate a reasonable contract and can hit deadlines.

"He never effs up," Poch says. "And he never gets mad — he never gets pissy."

At Vidcon, Graceffa and iJustine participate in a YouTube event called "brand speed-dating." About 10 brands — including Apple, Pizza Hut and Taco Bell — meet one-on-one with YouTube stars for about 10 minutes apiece.

For iJustine, it was especially thrilling to meet with Apple, Mandich says. She has long been obsessed with Apple products, though she has never taken a dime from the company. She has lined up to be the first to buy an iPhone, she has danced in Apple stores, she has gotten kicked out of a press event for the iPad, she has dressed up like Steve Jobs on the cover of her book. When Jobs died, she posted an emotional video of herself sobbing at the news — and caught flack for exploiting his death.

Apple has never used YouTube stars in its marketing. The idea that it might start was a sort of validation.

YouTube is still in its infancy, relatively speaking. And while it has created stars who draw audiences that dwarf those of premium cable shows, it's still anybody's guess how those stars cross over to mainstream fame.

Mandich and his colleagues are focused on what he calls "multi-threat talent" — vloggers who can do other things besides making videos. It's common for agents and managers who work in the YouTube arena to stress their clients' traditional skills.

"Most of these big digital stars are at their core storytellers," says Brent Weinstein, the head of digital media at UTA. "They're telling the stories of their lives and creating a connection between themselves and their passionate audiences. ... We are not as frequently referring to people as digital creators. They're simply artists."

Some YouTube stars have been on reality shows or played supporting roles on scripted shows. But they have not yet taken over Hollywood.

And the clock is ticking. Each year they get older, and it becomes a little harder to hang onto a very young audience.

"The industry that's growing around them really does not seem to care about the long-term welfare of these kids," says Lennon, Graceffa's former manager. "They're seen as quick money. Most of these talent have a lifespan of a few years unless they start utilizing the time when they're at the peak."

Mandich is trying to help his clients figure this out. Cat Valdes, 26, has been vlogging under the name Catrific since she was 18. She says that, while she has grown older and wiser, her character has stayed 18. Her goal is to age her persona, and her audience, without losing the subscribers she already has.

Most YouTube stars recognize that it's impolitic to talk openly about leaving the platform. They tend to express openness to traditional media opportunities, while saying they will always maintain their channels.

Though it looks tossed off, daily vlogging is hard work. Some managers say there's a risk of burnout.

Sitting in the Instagram lounge at Vidcon, iJustine is greeted by a bunch of other early YouTube vloggers. She gets out her Canon G7X video camera (Canon is a sponsor) and shoots a quick video. She gets excited when another vlogger has the same model camera.

Reminiscing with old friends, she's in a wistful mood. Unprompted, she says, "I'll stop doing it when it stops being fun."

"When you have millions of followers, you do think you can do anything," Lennon says. "You think, 'I'm gonna be making movies because I'm already famous, and that's what happens when you're famous.' They thought everyone in Hollywood was going to come to them, and they didn't need someone to open doors."

That hasn't happened, she says. "Hollywood hates these kids."

For his part, Mandich is busy minting new stars. After Vidcon, he flew to New York to kick off a six-city book tour for Dulce Candy, a beauty vlogger with a growing audience of Latino girls. L.A. was the fourth stop, and Mandich was there, shepherding her family around and making sure everything ran smoothly. He looked a little wrung out.

"I'm so tired," he said.

Lennon also is scouting fresh talent. She recently met an 11-year-old in Florida who wants Lennon to manage her. Lennon encouraged her to get into acting and singing classes.